Switch base-entity $id from HTTP URL to URN scheme

[mgoldsborough]

Problem

The base entity schema declares its $id as:

https://upjack.dev/schemas/v1/upjack-entity.schema.json

and every app entity schema references it via allOf: [{$ref: "https://upjack.dev/..."}].

That URL is a name, not a location — there's no guarantee the schema is actually served there. But because the $id looks like an HTTP URL, every JSON-Schema-aware consumer (FastMCP, validators, third-party tools) is justified in trying to dereference it over the network. This caused the exact bug fixed in 0.5.0: tools/list was blocking for ~4s per activity-enabled server while FastMCP's serializer tried to fetch the URL.

The 0.5.0 fix inlines the $ref at load_schema time so library-internal flows never re-resolve. That's a workaround. The root cause is that we're using a URL as an identifier.

Proposal

Switch the base-entity $id and the canonical $ref target to a URN:

{
  "$id": "urn:upjack:schemas:v1:entity"
}

URNs are, by spec, non-dereferenceable — consumers can't accidentally try to HTTP-fetch them. Any tool that wants to resolve the schema has to bring its own local copy, which is exactly the contract Upjack ships anyway (bundled upjack-entity.schema.json).

Consequences

• Breaking change to the published schema contract — existing app schemas reference the HTTPS URL.
• Target 0.6.0.
• Migration: app-schema authors update one string. The library continues to bundle the schema under the new \$id and auto-inline it at load time.
• The load_schema / `_inline_base_entity_ref` machinery stays — same mechanism, different URI.
• Follow-up: consider whether resolve_entity_schema helper (or a codemod in the app-builder skill) should help authors migrate.

Out of scope

• Hosting the schema somewhere real (e.g., on schemas.nimblebrain.ai) — the whole point of this change is that the schema doesn't need to be hosted.
• Generalizing to arbitrary $id URLs in user-authored schemas — those are the author's responsibility.

Related

• Fixed in 0.5.0 via inlining: Flatten entity CRUD tool inputs; inline base-entity $ref (0.5.0) #22